The invention relates to a device for rotating the plane of polarization of linearly polarized light in the form of a ball lens using magnetic crystalline material preferentially magnetized in the direction of light transmission.
The invention also relates to a method of fabricating such a device and to its application.
Devices made from magnetic crystalline material for rotating the plane of polarization of linearly polarized light are generally referred to as Faraday rotators.
In optical communication systems operating with optical single-mode waveguides and a light source (e.g. a laser diode), passive components are required such as for example lenses for focusing the entering light beam, or for example optical isolators incorporating a Faraday rotator, which transmit the light only in one direction, while optically blocking the opposite direction. Such isolators find application for protecting the light source, e.g. a laser, against interfering reflections from the connected optical system.
The present invention relates to the combination of two passive components used among others for the construction of an optical communication system with singlemode waveguides and a light source (in the form of for example a laser diode), namely a combination of a focusing ball lens and a Faraday rotator for an optical isolator.
A combination of these functions in one component is known from Electronics Letters 18 (1982), No. 24, pages 1026 to 1028, which proposes a ball lens of yttrium-iron-garnet (YIG) not only as a Faraday rotator but also as a coupling lens in an optical communication system.
Ball lenses of YIG can only be made at relatively high cost, since in the first place they require the growth of large single-crystals from which the balls then have to be ground. A further drawback is that yttrium-iron-garnet has a relatively small Faraday rotation; a ball lens used for example to transmit a wavelength .lambda.=1.3 .mu.m requires a ball diameter of 2.1 mm and thus requires considerable outlay to match it to the dimensions of optical waveguides used at the present time. Yet another disadvantage is that a relatively high magnetic field is required for the saturation magnetization of YIG.
Faraday rotators made from magnetic garnet material such as yttrium-gallium-iron-garnet Y.sub.3 (Ga, Fe).sub.5 O.sub.12 or from bismuth-substituted rare-earth metal iron garnet, for example (Gd, Bi).sub.3 (Ga, Fe, Al).sub.5 O.sub.12, are characterized by a high Faraday rotation .theta..sub.F (.degree./cm) combined with low optical losses in the near infrared. The Faraday rotation in the case of bismuth-substituted iron garnets is considerably larger than that of YIG (in this connection reference is made for example to G. Winkler, Magnetic Garnets, Vieweg, Braunschweig 1981, pages 253 et seq.).
These crystals have the disadvantage, however, that their Faraday rotation, owing to dispersion, is dependent on the wavelength of the incoming light. This means, for example, for an isolator, that the damping in the blocked direction, that is to say the ability to suppress interfering reflections from an optical system, is wavelength-dependent. If a high damping in the blocked direction is required, relatively little variation of the wavelength is possible. Consequently, the Faraday rotator, and hence the optical isolator built with it, can only be used in a spectral range of limited bandwidth. Since the spectral region in the near infrared from about 0.8 to 1.6 .mu.m has acquired considerable importance for optical data transmission using optical waveguides, this is a particularly serious drawback.